1.

INTRODUCTION

When deciding whether to develop a field, a company must estimate how much oil and gas will be recovered
and how easily they will be produced. Although the volume of oil and gas in place can be estimated from the
volume of the reservoir, its porosity, and the amount of oil or gas in the pore spaces, only a proportion of
this amount will be recovered. This proportion is the recovery factor, and is determined by various factors
such as reservoir dimensions, pressure, the nature of the hydrocarbon, and the development plan.
The advantages of wireline logging are considerable. It allows the acquisition of valuable data at a fast rate
and over a wide range of depths. This allows fast and accurate decisions to be made regarding drilling, based
on the information obtained. Understanding the physical properties of an oil well is critical to properly
managing it over its lifetime. Wireline logging makes that possible.

In wireline logging time is a critical factor. The cost of running operations on an offshore drilling rig is very
high: drilling a well might cost $12 million per day of opera ons. In such opera ons, down me and logging
equipment failures are expensive. Well logging equipment costs are only a small part of the cost of drilling
operations and generally a very small fraction of the hydrocarbon production costs. Modifications that
improve the accuracy of logging without compromising reliability of the data are welcome in the industry
even if they raise the cost. As a result, many techniques have been used for well logging. Several techniques
are discussed in this report.

Well loggers use combinations of both radiation-based and non-radiation-based tools (called nuclear and
nonnuclear in this field) to examine the earth formations surrounding the well and sensors to detect the
medias response to interrogation tools. An analyst examines detector logs to look for some or all of the
following parameters of the formation: formation water saturation, porosity, rock characteristics,
carbon/oxygen ratio, and permeability.
Because of the complexity of earth formations, only a combination of all the logs allows the log analyst to
draw accurate conclusions for the formation parameters. For example, combining resistivity and nuclear
logs, the log analyst can determine porosity, water content, and density (see g(1.1)).

Fig(1.1) Typical logs and evalua on results from a string of well logging tools, including a natural gamma
-ray
log, a neutron log, an array induction log, and resistivity logs. The hydrocarbon volume result is also shown
at the right

1.1

Definition of wire line logging

Well logs result from a probe lowered into the borehole at the end of an insulated cable. The resulting
measurements are recorded graphically or digitally as a function of depth. These records are known as
geophysical well logs, petrophysical logs, or more commonly well logs, or simply logs.

Wireline logging has a history that goes back just over 80 years to September 5th, 1927 when two
brothers, Conrad and Marcel Schlumberger, ran what is considered to be the very first wireline log at
the Pechelbronn Oil Company oil field in France.
Their experimental logging attempt was a success and the brothers called their new technique an
electric survey. A few years later in the early 1930s in the USA the term "well log" was being used.
Wireline logging is so called because the logging tool is lowered through the oil well or borehole on
the end of a wireline.

1.2

Main principle:

The sensing element is fixed on the sonde, this element gathers information from the well and then entered
to a signal conditioning element to make the signal ready for transmission.The data from the sonde are
transmitted up the cable to instruments in the logging truck where they are recorded (field print). The data
are also processed later, and a cleaner log (final print) is made. The logging data are digitised (if was not
digital already), recorded on the hard drive, and sent to a logging company office (email), otherwise put on a
server or the Internet.

1.3

How to make a wireline well log:

To make a wireline well log after the well (a section) is drilled (and before setting casing), the hole is first
cleaned by the circulating drilling mud and then the drilling equipment is pulled from the well.
Then a sonde (probe) is lowered down the well (which is still filled with the drilling mud) on a logging cable.
The logging cable is an armoured cable with steel cables surrounding conductor cables in insulation. It is
reeled out from the drum in the back of a logging truck.

1.4

Sonde

The sonde or tool is a cylinder, commonly 27 to 60

(8 to 19 m) long and some mes up to 90
(27.5m) long, 3 to 4 in (8 to10 cm) in diameter and is lled with instruments (electric, nuclear or
acoustic transmitters, receivers and amplifiers).

Several instrument packages such as formation density, neutron porosity and gamma ray can be
screwed together to form the sonde.

The sonde has either one expandable arm or bow spring that puts the sensors in contact with the
well walls or three expandable arms or bow springs that centers the sonde in the well.

As the sonde is run back up the well, it remotely (with respect to a guy in the truck) senses the
electrical, acoustical, and/or radioactive properties of the rocks and their fluids and sometimes the
geometry of the wellbore.

In a directional well with a high deviation or a horizontal hole, the sonde must be pushed down the
well with tubing or the drillstring. One trip down and up with a sonde is called a run.

1.4.1

Main components of the sonde:

Sonde on a wireline
a) cross section of the
armoured cable,
b) Sondes with arms,
c) Sondes with bow spring(s)

2. SPONTANEOUS POTENTIAL LOG

The spontaneous potential log, commonly called the self-potential log or SP log, is a measurement
taken by oil industry well loggers to characterise rock formation properties. The log works by
measuring small electric potentials (measured in millivolts) between depths in the borehole and a
grounded voltage at the surface.
Its one of the first log measurements made. It was discovered as a potential that effected old electric
logs .It has been in use for over the past 50 years.
The change in voltage through the well bore is caused by a buildup of charge on the well bore walls.
Clays and shales (which are composed predominantly of clays) will generate one charge and
permeable formations such as sandstone will generate an opposite one. This build up of charge is, in
turn, caused by differences in the salt content of the well bore fluid (drilling mud) and the formation
water (connate water). The potential opposite shales is called the baseline, and typically shifts only
slowly over the depth of the borehole. Whether the mud contains more or less salt than the connate
water will determine the which way the SP curve will deflect opposite a permeable formation. The
amplitudes of the line made by the changing SP will vary from formation to formation and will not
give a definitive answer to how permeable or the porosity of the formation that it is logging.

2.1 APPLICATIONS
The SP tool is one of the simplest tools and is generally run as standard when logging a hole, along
with the gamma ray. SP data can be used to find:

Correlation from well to well .

Depth reference for all logging runs .
Detecting permeable beds (Where the permeable formations are ).
The boundaries of these formations Detecting bed boundaries .
Rw determination and the values for the formation-water resistivity .

The SP curve can be influenced by various factors both in the formation and introduced into the
wellbore by the drilling process. These factors can cause the SP curve to be muted or even inverted
depending on the situation.

Bed thickness (h), and true resistivity (Rt) of the permeable bed.
Invaded resistivity (Rxo) and the diameter of invasion (di)
Ratio of mud filtrate to formation water salinities - Rmf/Rw
Neighboring shale resistivity (Rs)
Hole diameter (dh)
Mud resistivity (Rm)

There are many SP correction charts available although no one chart is able to include all the
possible variables in making the necessary corrections.

Fig(2.1): SP Correction Chart

The drilling mud salinity will affect the strength of the electromotive forces (EMF) which give the
SP deflections. If the salinity of the mud is similar to the formation water then the SP curve may give
little or no response opposite a permeable formation; if the mud is more saline, then the curve has a
positive voltage with respect to the baseline opposite permeable formations; if it is less, the voltage
deflection is negative. In rare cases the baseline of the SP can shift suddenly if the salinity of the mud
changes part way down hole.
Mud invasion into the permeable formation can cause the deflections in the SP curve to be rounded
off and to reduce the amplitude of thin beds. A larger wellbore will cause, like a mud filtrate
invasion, the deflections on the SP curve to be rounded off and decrease the amplitude opposite thin
beds, while a smaller diameter wellbore has the opposite effect.

Illustration of the principle of the spontaneous potential (SP) log. A natural potential is measured
between an electrode in the well and earth at the surface (redrawn from Rider, 1996).
The SP electrode is built into different logging tools for example:
Induction log.
Laterolog.
Sonic log.
Sidewall core gun.

Fig(2.2): Borehole mud invasion profile

Fig (2.3): The SP measurement

SP results from electric currents flowing in the drilling mud. There are three sources of the
currents, two electrochemical and one electrokinetic. Deflection of SP is caused by the
Electrochemical Ec and Electrokinetic Ek actions:

2.2 Electrochemical Component

Ec = Elj + Em
These two effects are the main components of the SP. They are caused as a result of differing
salinities in the mud filtrate and the formation water.
Elj: "Liquid Junction Potential"
The ions Na+ and Cl- have different nobilities at the junction of the invaded and virgin zones.
The movement of the ions across this boundary generates a current flow and hence a
potential.
If the salinity of the mud in the borehole is weaker or stronger than that of the formation
water the potential generated between the two solutions is known as the Liquid Junction
Potential or Elj. The greater the difference between the salinity of the solutions the greater the
potential.

Fig(2.4): Liquid Junction Effects #1

Fig(2.5): Liquid Junction Effects #2

Em: "Membrane Potential"

Shales are permeable to Sodium ions but not to Chlorine ions. Hence there is a movement of
charged particles through the shale creating a current and thus a potential. This is known as
the membrane potential or Em.

Fig (2.6): Membrane Potential SP

2.3 Deflection of the SP curve

The SP measurement is constant but jumps suddenly to another level when crossing the
boundary between two different formations.
When Rmf > Rw The SP deflects to the left (-ve SP) found in permeable formations filled
with formation water ,
When Rmf < Rw The SP deflects to the right (+ve SP) found in permeable formation filled
with formation water ,
There is no deflection in non-permeable or shaly formations.

Fig(2.7): SP Deflection

2.4 CALIBRATION
In the logging unit there is a small battery and a potentiometer in series between the two electrodes.
The logging engineer can adjust the potentiometer so that the SP appears in track 1. Since we need to
remove all extraneous potentials to the membrane potential, the SP needs to be normalised in a
computing centre so that there is no potential (SP=0.0MV) opposite shale beds. This is done
concurrently with the SP drift correction. The absolute difference between shale and sand remains
the same after drift correction. Caution:
Some field engineers in the past varied the potentiometer to correct the drift while logging and
therefore keep the SP on the display track. Recent logging tools record the raw SP on data storage
(i.e. no battery and no potentiometer) and it is sometimes preferable to use this raw SP to perform the
SP correction. An offset can be applied to the raw SP if its values range significantly above zero.

2.5 LIMITATIONS :

Borehole mud must be conductive.

Formation water must be water bearing and conductive.
A sequence of permeable and non-permeable zones must exist.
Small deflection occurs if Rmf=Rw .
Not fully developed in front of thin beds .

2.6 Metallic reaction at measure electrode

This is one of the components that will cause the SP to drift. The SP electrode made of mild
iron will rust and this oxidizing effect of the electrode results in an added electrochemical
potential to the SP measurement. The drift gradually disappears as the electrode becomes
fully oxidized. Because this is an undesirable potential, the drift can be removed by
correcting the SP curve using computer software.
Possible solution to the problem:

The bridle electrode should be made of lead as it incurs less oxidization and
therefore less drift.
Never clean or remove the rust from the SP electrode.
One hour before going down hole, wrap the electrode in a rag soaked in the mud
pit. This will reduce the oxidizing effect down hole .

2.7 Other unwanted SP potentials

Heavy rain:
If heavy rain starts during logging, the surface conductivity of the soil will gradually change
and therefore can gradually change the potential between the surface reference and the down
hole electrode and thus contribute to the SP drift.
Noise:
Surface noise such as electrical leakages on the rig, welding equipment, weather storms and
lightning strikes will cause the SP to be noisy and at random. No welding should be allowed
during the recording of the SP log.
Logging drum and sheave magnetism:
If part of the logging drum, wire line sheave or measure wheel is magnetized, this will appear
on the SP curve as a short and regular deflections.
Disruptions to the ground reference:
The SP electrode (called the fish) should be placed in an undisturbed position in the mud pit
away from moving mud fluids.
Power lines, electric trains, close radio transmitters and cathodic protection devices all create
currents, which disrupt the ground electrode reference causing a poor, sometimes useless log.
Bimetallism occurs when two different metals are touching surrounded by mud produces a
weak battery.

3. ACOUSTIC LOGGING

3.1 Definition in Geophysics:

A display of travel time of acoustic waves versus depth in a well. The term is commonly used as a synonym
for a sonic log. Some acoustic logs display velocity.

3.2 Formation evaluation:

A record of some acoustic property of the formation or borehole. The term is sometimes used to refer
specifically to the sonic log, in the sense of the formation compressional slowness. However, it may also
refer to any other sonic measurement, for example shear, flexural and Stoneley slownesses or amplitudes,
or to ultrasonic measurements such as the borehole televiewer and other pulse-echo devices, and even to
noise logs.

3.3 Introduction

The sonic or acoustic log measures the travel time of an elastic wave through the formation. This
information can also be used to derive the velocity of elastic waves through the formation. Its main use is to
provide information to support and calibrate seismic data and to derive the porosity
of a formation. The main uses are:
Provision of a record of seismic velocity and travel time throughout a borehole. This information
be used to calibrate a seismic data set (i.e., tie it in to measured values of seismic velocity).
Provision of seismic data for the use in creating synthetic seismograms.
Determination of porosity (together with the FDC and CNL tools).
Stratigraphic correlation.
Identification of lithologies.
Facies recognition.
Fracture identification.
Identification of compaction.

can

Identification of over-pressures.
Identification of source rocks.
The tool works at a higher frequency than seismic waves, therefore one must be careful with the direct
comparison and application of sonic log data with seismic data.

3.4 Wave Types

The tool measures the time it takes for a pulse of sound (i.e., and elastic wave) to travel from a transmitter
to a receiver, which are both mounted on the tool. The transmitted pulse is very short and of high
amplitude. This travels through the rock in various different forms while undergoing dispersion (spreading of
the wave energy in time and space) and attenuation (loss of energy through absorption of energy by the
formations). When the sound energy arrives at the receiver, having passed through the rock, it does so at
different times in the form of different types of wave. This is because the different types of wave travel with
dierent veloci es in the rock or take dierent pathways to the receiver. Figure 16.1 shows a typical
received train of waves. The transmitter fires at t = 0. It is not shown in the gure because it is masked from
the received information by switching the receiver off for the short duration during which the pulse is
transmitted. This is done to ensure that the received information is not too complicated, and to protect the
sensitive receiver from the high amplitude pulse. After some time the first type of wave arrives. This is the
compressional or longitudinal or pressure wave (P-wave). It is usually the fastest wave, and has a small
amplitude. The next wave, usually, to arrive is the transverse or shear wave (S- wave). This is slower than the
P-wave, but usually has a higher amplitude. The shear wave cannot propagate in fluids, as fluids do not
behave elastically under shear deformation. These are the most important two waves. After them come
Rayleigh waves, Stoneley waves, and mud waves. The first two of these waves are associated with energy
moving along the borehole wall, and the last is a pressure wave that travels through the mud in the
borehole. They can be high amplitude, but always arrive after the main waves have arrived and are usually
masked out of the data. There may also be unwanted Pwaves and S-waves that travel through the body of
the tool, but these are minimized by good tool design by (i) reducing their received amplitude by arranging
damping along the tool, and (ii) delaying their arrival until the P-wave and S-wave have arrived by ensuring
that the pathway along the tool is a long and complex one. The data of interest is the time taken for the Pwave to travel from the transmitter to the receiver. This is measured by circuitry that starts timing at the
pulse transmission and has a threshold on the receiver. When the first P-wave arrival appears the threshold
is exceeded and the timer stops. Clearly the threshold needs to be high enough so that random noise in the
signal dies not trigger the circuit, but low enough to ensure that the P-wave arrival is accurately timed.

Fig (3.1) The geophysical wavetrain received by a sonic log.

There are complex tools that make use of both P-waves and S-waves, and some that record the full wave
train (full waveform logs). However, for the simple sonic log that we are interested in, only the first arrival of
the P-wave is of interest. The time between the transmission of the pulse and the reception of the first
arrival P-wave is the one-way time between the transmitter and the receiver. If one knows the distance
between the transmitter (Tx) and the receiver (Rx), the velocity of the wave in the formation opposite to the
tool can be found.
In practice the sonic log data is not presented as a travel time, because different tools have different Tx-Rx
spacings, so there would be an ambiguity. Nor is the data presented as a velocity. The data is presented as a
slowness or the travel time per foot traveled through the formation, which is called delta t (t or T), and is
usually measured in s/ft. Hence we can write a conversion equation between velocity and slowness:

where the slowness, t is in microseconds per foot, and the velocity, V is in feet per second.
The velocity of the compressional wave depends upon the elastic properties of the rock (matrix plus fluid),
so the measured slowness varies depending upon the composition and microstructure of the matrix, the
type and distribution of the pore fluid and the porosity of the rock. The velocity of a Pwave in a material is
directly proportional to the strength of the material and inversely proportional to the density of the
material. Hence, the slowness of a P-wave in a material is inversely proportional to the strength of the
material and directly proportional to the density of the material, i.e.;

The strength of a material is defined by two parameters (i) the bulk modulus, and (ii) the shear modulus.

3.5 Reflection and Refraction

The transmi er emits sound waves at a frequency of about 20-40 kHz, in short pulses, of which there are
between 10 and 60 per second depending on the tool manufacturer. The energy spreads out in all directions.
Imagine a pulse emanating from a Tx on a sonic tool. It will travel through the drilling mud and encounter
the wall of the borehole. The P-wave travels well through the mud at a relatively slow velocity, Vm, as the
mud has a low density. The S-wave will not travel through liquid mud. At the interface it is both reflected
back into the mud and refracted into the formation. The portion of the Pwave energy that is refracted into
the formation travels at a higher velocity, Vf, because the density of the rock is higher. We can use Snells
law to write;

and at the critical angle of refraction, where the refracted wave travels along the borehole wall, R= 90o, so;

Hence, if the velocity of the elastic wave in the formation changes, the critical angle, i, will also change.
The velocity of the refracted wave along the borehole wall remains Vf. Each point reached by the wave acts
as a new source retransmitting waves back into the borehole at velocity Vm.

Fig (3.2) Reec

on andrefraction at the borehole wall

3.6 Sonic Tools

3.6.1 Early Tools

Early tools had one Tx and one Rx. The body of the tool was
made from rubber (low velocity and high attenuation material)
to stop waves travelling preferentially down the tool to the Rx.
There were two main problems with this tool. (i) The measured
travel time was always too long because the time taken for the
elastic waves to pass through the mud was included in the
measurement. The measured time was A+B+C rather than just
B. (ii) The length of the formation through which the elastic
wave traveled (B) was not constant because changes to the
velocity of the wave depending upon the formation altered the
critical refraction angle.
Fig(3.3 ) Early sonic tools.
3.6.2 Dual Receiver Tools

These tools were designed to overcome the problems in the early tools. They use two receivers a few feet
apart, and measure the difference in times of arrival of elastic waves at each Rx from a given pulse from the
Tx . This time is called the sonic interval transit time (t) and is the time taken for the elastic wave to travel
through the interval D (i.e., the distance between the receivers).
The

me taken for elas

The time taken for elas

c wave to reach Rx1:

TRx1= A+B+C
c wave to reach Rx2:TRx2 = A+B+D+E

The sonic interval transit time: T = (TRx2 - TRx1) = A+B+D+E (A+B+C) = D+E-C.
If tool is axial in borehole: C = E, so T = (TRx2 - TRx1) = D
The problem with this arrangement is that if the tool is tilted in the hole, or the hole size changes, we can
see that C E, and the two Rx system fails to work.

Fig (3.4)Dual receiver sonic tools in correct

Fig (3.5) Dual receiver sonic tools in incorrect

configuration.

configuration.

3.6.3 Borehole Compensated Sonic (BHC) Tool

This tool compensates automatically for problems with tool
misalignment and the varying size of the hole (to some extent) that
were encountered with the dual receiver tools. It has two
transmitters and four receivers, arranged in two dual receiver sets,
but with one set inverted (i.e., in the opposite direction). Each of
the transmitters is pulsed alternately, and t values are measured
from alternate pairs of receivers. These two values of t are then
averaged to compensate for tool misalignment, at to some extent
for changes in the borehole size.
A typical pulse for the BHC is 100 s to 200 s, with a gap of about
50 ms, giving about 20 pulses per second. There are four individual
Tx-Rx readings needed per measurement, so 5 measurements can
be made per second. At a typical logging speed of 1500 m/h (5000
/h), gives one reading per 8 cm (3 inches) of borehole. Several
versions of the BHC are available with different Tx-Rx distances
(3

. and 5

. being typical),and the Rx-Rx distance between

Fig(3.6) Borehole compensated sonic tools.

pairs of receivers is usually 2

3.6.4 Long Spacing Sonic (LSS) Tool

It was recognized that in some logging conditions a longer Tx-Rx

distance could help. Hence Schlumberger developed the long spacing
sonic (LSS), which has two Tx two feet apart, and two Tx also two feet
apart but separated from the Tx by 8 feet. This tool gives two
readings; a near reading with a 8-10 . spacing, and a far reading with
a 10-12 . spacing.

Fig(3.7) Long spacing sonic tools.

3.7 Calibration

The tool is calibrated inside the borehole opposite beds of pure and known lithology, such as anhydrite (50.0
s/ .), salt (66.7s/ .), or inside the casing (57.1s/ft.).

3.8 Depth of Investigation

This is complex and will not be covered in great detail here. In theory, the refracted wave travels along the
borehole wall, and hence the depth of penetra on is small (2.5 to 25 cm). It is independent of Tx- Rx spacing,
but depends upon the wavelength of the elastic wave, with larger wavelengths giving larger penetrations. As
wavelength l = V/f (i.e., velocity divided by frequency), for any given tool frequency, the higher the velocity
the formation has, the larger the wavelength and the deeper the penetration.

3.9 Logging Speed

The typical logging speed for the tool is 5000

/hr (1500 m/hr), although it is occasionally run at

lower speeds to increase the vertical resolution.

The Compensated Sonic Sonde

The Long Spaced Compensated Sonic Sonde

4. GAMMA RAY LOG

4.1 Introduction

There are a number of nuclear well logging tools that have been and still are important in the evaluation of
hydrocarbon wells and reservoirs. While the recent interest in logging-while-drilling tools has changed the
emphasis somewhat, interest in nuclear tools has remained as high as, or higher than, ever.
The nuclear tools play roles in the determination of a number of the most important hydrocarbon well
characteristics such as porosity, elemental composition, and whether or not oil or water is present. The
nuclear tools of primary interest use either sources of gamma rays or neutrons.

4.2 Definition:

Is a wireline well log that records the natural radio activity (gamma ray emission) of rocks in the well by a
scintillation crystal in the snode.

4.3 Basic principle:

The Gamma ray tool produces a measurement of the naturally occurring

radiation found in rock formations. The Gamma Log produced by these tools
is commonly used for depth correction, correlation with open hole logs and
identifying low radiation and high radiation lithologies.

4. 4 Types of Gamma probes:

1. Total count probes ( measures the concentration of Gamma rays).

2. Spectral probes ( measures the energy of each gamma ray).

4.5 Method of operation:

Natural gamma-ray tools are designed to measure naturally occurring gamma radiation in the earth
caused by the disintegration due to Potassium, Uranium, and Thorium. Unlike nuclear tools, these
natural gamma ray tools do not emit any radiation.
Natural gamma ray tools employ a radioactive sensor, which is usually a scintillation crystal that
emits a light pulse proportional to the strength of the gamma ray pulse incident on it. This light pulse
is then converted to a current pulse by means of a photo multiplier tube PMT where the current is
amplified about 1x106 times. From the photo multiplier tube, the current pulse goes to the tool's
electronics for further processing and ultimately to the surface system for recording. The data then
can be converted to energy spectra which can be easily read to find information about the well. The
strength of the received gamma rays is dependent on the source emitting gamma rays, the density of
the formation, and the distance between the source and the tool detector.

4.6 Main difference between neutron method and gamma ray method :

The natural gamma-ray tool has no source and detects the natural gamma rays that are present in the
rock formation outside the borehole.
A datasheet of a gamma probe is given at appendix A.

5. NEUTRON LOGGING
5.1 Definition
Neutron tools were the first logging instruments to use radioactive sources for determining the
porosity of the formation.
Neutron tool response is dominated by the concentration of hydrogen atoms in the formation. In
clean reservoirs containing little or no shale, the neutron log response will provide a good measure of
formation porosity if liquid-filled pore spaces contain hydrogen, as is the case when pores are filled
with oil or water (hydrogen index =1). By contrast, when logging shale or gas-bearing formations, a
combination of Neutron and Density readings will often be required for accurate porosity
assessment.
5.2 Basic principles
The electrically neutral neutron has a mass that is
practically identical to that of the hydrogen atom. The
neutrons that are emitted from a neutron source have a
high energy of several million electron volts (MeV).
After emission, they collide with the nuclei within the
borehole fluid and formation materials. With each
collision, the neutrons loose some of their energy. The
largest loss of energy occurs when the neutrons collide
with hydrogen atoms. The rate at which the neutrons
slow-down depends largely on the amount of hydrogen
in the formation.
With each collision the neutrons slow down, until the
neutrons reach a lower (epithermal) energy state and
then continue to lose energy until they reach an even lower (thermal) energy state of
fig . (5.1) General neutron logging tool
About 0.025 eV. At this energy the neutrons are in thermal equilibrium with other nuclei in the
formation. At thermal speeds, the neutrons will eventually be captured by a nucleus. When nucleus
captures a thermal neutron, a gamma ray (called a gamma ray of capture)is emitted to dissipate
excess energy within the atom.
The amount of energy lost at each collision depends on the relative mass of the target nucleus, and
the scattering cross section. (At the nuclear level, the term cross section is defined as the effective
area within which a neutron must pass in order to interact with an atomic nucleus. Such interactions
are typically classified either as neutron capture or as neutron scatter. The cross-section is a
probabilistic value dependent on the nature and energy of the particle, as well as the nature of the
capturing or scattering nucleus.

Depending on the type of tool being

used, either the gamma rays emitted
after neutron capture, the epithermal
neutrons or the thermal neutrons will
be counted.

Fig.(5.2) Emission, traveling & collisions of neutrons in

formation

The principles of neutron logging are summarized below:

A neutron source emits a continuous flux of high-energy neutrons.

Collisions with formation nuclei reduce the neutron energy -thereby slowing it down.

At thermal energy levels (approximately 0.025 eV), neutrons are captured.

Neutron capture results in an emission of gamma rays.

Depending on the type of tool, the detector measures the slowed down neutrons and/or
emitted gamma rays.
Neutron logging devices contain one or more detectors and a neutron source that continuously emits
energetic (fast) neutrons.

Fig.(5.3) Slowing down power of H, O, SI for different neutron energies

Fig.(5.4) Neutron energy level versus time after leaving the source illustrates the slow down
process

Porosity (or the hydrogen index) can be determined by measuring epithermal or thermal neutron
populations, or by measuring capture gamma rays, or any combination thereof.
Neutron logs that detect epithermal neutrons are referred to as sidewall neutron logs. By contrast,
the compensated neutron log, in widespread use today, detects thermal neutrons, using two
neutron detectors to reduce borehole effects. Single thermal neutron detector tools, of poorer
quality, are also available in many areas of the world.
Capture gamma rays are used for porosity determination, and logs of this type are referred to as
neutron-gamma logs. The responses of these devices are dependent upon such variables as
porosity, lithology , hole size, fluid type, and temperature.
Compensated and sidewall logs use corrections from their electronic panels to account for some of
these variables, while neutron-gamma logs require departure curves (provided in chart books) to
make corrections.

Example :
given that the lithology is dolomite with apparent porosity 15% ,which read directly from a sidewall
neutron porosity log (SNP) ,first find the apparent porosity along the scale at the bottom of the
correction chart ,then follow the line vertically until it intersect the curve representing dolomite,
finally read the true porosity on the left hand scale ,12% .

5.3 Combination Neutron Density Log

The Combination Neutron-Density Log is a combination porosity log. Besides its use a porosity
device, it is also used to determine lithology and to detect gas-bearing zones. The Neutron-Density
Log consists of neutron and density curves recorded in tracks #2 and #3 and a caliper and gamma ray
log in track #1. Both the neutron and density curves are normally recorded in limestone porosity
units with each division equal to either two percent or three percent porosity; however, sandstone and
dolomite porosity units can also be recorded.
Where an increase in density porosity occurs along with a decrease in neutron porosity in a gasbearing zone, it is called gas effect. Gas effect is created by gas in the pores. Gas in the pores causes
the density log to record too high a porosity (i.e. gas in lighter than oil or water), and causes the

neutron log to record too low a porosity (i.e. gas has a lower concentration of hydrogen atoms than
oil or water). The effect of gas on the Neutron-Density Log is a very important log response because
it helps a geologist to detect gas-bearing zones.
5.4 Gamma Ray-Sonic-DensityNeutron combinations
The gamma ray log measures the natural radiation of a formation, and primarily functions as a
lithology log. It helps differentiate shales (high radioactivity) form sands, carbonates, and anhydrites
(low radioactivity). The neutron log is a porosity device that is used to measure the amount of
hydrogen in a formation. The density log is a porosity device that measures electron density. When
these three logs are used together (i.e. Combination Gamma Ray Neutron-Density log), lithologies
can be determined.

5.5 NEUTRON LOGGING APPLICATIONS

Neutron tools are used primarily to determine:

Porosity, usually in combination with the density tool

Gas detection, usually in combination with the density tool, but also with a sonic tool

Shale volume determination, in combination with the density tool

Lithology indication, again in combination with the density log and/or sonic log

Formation fluid type.

Depending on the device, these applications may be made in either open or cased holes.
Additionally, because neutrons are able to penetrate steel casing and cement, these logs can be used
for depth tie-in as well as providing information on porosity and hydrocarbon saturations in cased
holes
An example of such a tool is API string tool from schlumberger (down, right) ,you can find more in
appendix B.

6. RESISTIVITY LOGS

6.1 Introduction
Electrical resistivity is a fundamental geophysical method used in both SURFACE and
SUBSURFACE geophysics. The method is legendary among Geophysical methods for exploration,
development and definition of existing targets .
Electrical resistivity is popular because it is a simple, low cost and efficient method. It is without
doubt the most practical, cost-effective logging method available today.
Most rock materials are essentially insulators, while their enclosed fluids are conductors.
Hydrocarbon fluids are an exception, because they are almost infinitely resistive. When a formation
is porous and contains salty water, the overall resistivity will be low. When the formation contains
hydrocarbon, or contains very low porosity, its resistivity will be high. High resistivity values may
indicate a hydrocarbon bearing formation.
A log of the resistivity of the formation, expressed in ohm-m. The resistivity can take a wide range of
values, and, therefore, for convenience is usually presented on a logarithmic scale from, for example,
0.2 to 2000 ohm-m. The resistivity log is fundamental in formation evaluation because hydrocarbons
do not conduct electricity while all formation waters do. Therefore a large difference exists between
the resistivity of rocks filled with hydrocarbons and those filled with formation water. Clay minerals
and a few other minerals, such as pyrite, also conduct electricity, and reduce the difference. Some
measurement devices, such as induction and propagation resistivity logs, may respond more directly
to conductivity, but are presented in resistivity.
6.2 Definition
By definition, resistivity is a function of the dimensions of the material being measured; therefore, it
is an intrinsic property of that material. Resistivity is defined by the formula:
Where Electrical resistivity is defined by:

Where

Fig(6.1)

is the static resistivity (measured in volt-metres per ampere, Vm/A);

E is the magnitude of the electric field (measured in volts per metre, V/m);
J is the magnitude of the current density (measured in amperes per square metre, A/m).

The electrical resistivity (rho) can also be given by,

where
is the static resistivity (measured in ohm-metres, m);
R is the electrical resistance of a uniform specimen of the material (measured in ohms, );
is the length of the piece of material (measured in metres, m);
A is the cross-sectional area of the specimen (measured in square metres, m).
Finally, electrical resistivity is also defined as the inverse of the conductivity (sigma), of the
material, or:

6.3 Method of operation

Resistivity logs measure some aspect of the specific resistance of the geologic formation. There are
about 17 types of resistivity logs, but they all have the same purpose which is to measure the electric
conductivity fluid in the rock. Electrical resistivity (also known as specific electrical resistance or
volume resistivity) is a measure of how strongly a material opposes the flow of electric current. A
low resistivity indicates a material that readily allows the movement of electrical charge.
In these logs, resistivity is measured using 4 electrical probes to eliminate the resistance of the
contact leads with 2 current electrodes and 2 measurement electrodes. The log must run in holes
containing electrically conductive mud or water .

6.4 Basic Principle:

The principles of measuring resistivity are illustrated in fig (6.2). If 1 amp of current from a 10-V
battery is passed through a 1-m3 block of material, and the drop in potential is 10 V, the resistivity of
that material is 10 Wm. The current is passed between electrodes A and B, and the voltage drop is
measured between potential electrodes M and N, which, in the example, are located 0.1 m apart-, so
that 1 V is measured rather than 10 V. The current is maintained constant, so that the higher the
resistivity between M and N, the greater the voltage drop will be. A commutated DC current is used
to avoid polarization of the electrodes that would be caused by the use of direct current.

Fig (6.2). Principles of measuring resistivity in Ohm-meter. Example is 10 Ohm-meter.

There are 3 different configurations of resistivity log:

short-normal:

has the smallest distance between 2 adjacent electrodes (40 cm (16 in or less)). It is
the most sensitive to thin layers but is also influenced by the drilling mud, short normal devices are
considered to investigate only the invaded zone
long-normal: long normal (162 cm (64 in)) devices are considered to investigate both the invaded
zone and the zone where native formation water is found
lateral: Lateral log has the longest distance between two adjacent electrodes (18 feet 8 inches). It
samples resistivity over a large section of sediment/rock away from the borehole. Lateral log may
miss thin beds.
6.5 Types of resistivity logs:
There are many different types of resisitivity logs, which differ primarily in how far into the rocks
they measure the resistivity. Because drilling fluids tend to force their way into the surrounding rock,
resistivity logs with shallow depths of investigation are unable to see beyond an "invasion zone" to
determine the true formation water resistivity of permeable rocks. Instead, these logs measure the
lower resistivity of the contaminated zone. Thus, by pairing logs with deep and shallow depths of
investigation, it is possible to measure permeability by looking at the resistivity diffences between
the logs. The acronyms of some of the more popular resistivity logs are listed below:

AIT (Array Induction Tool) - the resistivity log of the future. It measures five depths
of investigation.
DIL (Dual Indiction Log) - a frequently used log with deep and medium depths of
investigation.
DLL (Dual Laterolog) - a frequently used log with deep and medium depths of
investigation.
LAT (Lateral Log)- an obsolete log with a deep depth of investigation.
LN (Long Normal) - an obsolete log with a deep depth of investigation.
SFL (Spherically Focused Log) - a frequently used log with a shallow depth of
investigation.
SGR (Shallow Guard Log) - a frequently used log with a shallow depth of
investigation.
SN (Short Normal) - an obsolete log with a shallow depth of investigation.

6.6 Calibration
resistivity logging systems may be calibrated at the surface by placing fixed resistors between the
electrodes. The formula used to calculate the resistor values to be substituted in the calibration
network shown in fig(6.3) .

Figure( 6.3). System for calibrating resistivity equipment

Wireline Logging

Sukina Y. Bader
Lara Qasem
Amal Ryahe
Sara Naser
Hanan Ahyad

L o g g i n g

t o o l s

f o r

o i l

e x p l o r a t i o n

Gamma-Ray Tools
For Geosteering, MWD and Wireline Logging

Pa g e 1 o f 4

2601 McHale Court

Suite 145
Austin, Texas 78758

Tel: 512-491-7541
Fax: 512-491-7561
www.cbgcorp.com

Gamma-Ray Tools

For Geosteering, MWD and Wireline Logging

Gamma Tool Description

The Gamma ray tool produces a measurement of the naturally occurring
radiation found in rock formations. The Gamma Log produced by these
tools is commonly used for depth correction, correlation with open hole
logs and identifying low radiation and high radiation lithologies. CBG
Gamma ray tools use a super sensitive hermetically sealed Sodium Iodide
Scintillator crystal and a ruggedized high temperature photomultiplier for
maximum log quality. Mechanical design techniques have been developed
specically for the MWD/Steering tool environment to ensure a rugged and

reliable tool. The short single piece aluminum chassis not only provides
maximum strength and rigidity but minimizes vibration loads due to the
low mass. The electronics are fully temperature compensated to maintain
consistent count rates through the 350F temperature rating. The tool uses
a gross counting discriminator with an energy threshold set at approximately
15KeV, signicantly lower than other tools, resulting in higher count rates
and greater accuracy. CBG provides customized models of Gamma ray
tools for Geosteering and MWD.

NGT-T Gamma Tool

for Geosteering and MWD
The NGT-T Gamma tool has become the industry standard for Geosteering
and MWD applications. This tool was initially developed for the Steering tool
industry in 1994 and was later upgraded to meet the severe environmental
challenges of Measurement While Drilling. The standard model is equipped
with an MDM15pin male connector on the top electronics end of the tool
and an MDM15pin female connector on the bottom. This tool utilizes Pin#1
for Ground, Pin# 4 for Power and Pin# 8 for Signal. All 15 wires are passed
along a protected wire guide from top connector to bottom. Electronics
are encapsulated for additional protection. The crystal and photomultiplier
are packaged in house utilizing our proprietary, unique design for ease of
replacement or repair.

NGT-T Gamma Tool

with Pressure Housing Assembly
The NGT-T Gamma Tool can now be ordered to include the complete
Tensor Compatible mechanical assembly. The NGT-T is mounted to the
Bottom Bulkhead Retainer through a standard Shock Snubber Assembly. A
connector pigtail converts the MDM15pin connector on the tool to a 200C,
GE, 4Pin/6Socket connector mounted within the bottom Intermodule End.
At the top, a pigtail converts the MDM15pin connector on the NGT-T to
a 200C, GE, 6Pin/4Socket connector mounted within the top Intermodule
End. A custom 24 BeCu Pressure Barrel results in a signicantly shorter
and lower cost tool than was previously available to the market.

NGT-CS Tool
for Geosteering and MWD
The NGT-CS Gamma tool is the small diameter version of the popular
NGT-T. At just 1.05 OD, it offers the same performance and durability
of the NGT-T. A smaller diameter scintillator crystal with increased length
matches the sensitivity of the larger tool.

Pa g e 2 o f 4

2601 McHale Court

Suite 145
Austin, Texas 78758

Tel: 512-491-7541
Fax: 512-491-7561
www.cbgcorp.com

Surface of tool
4 inches of radius

CBG Azimuthal
Gamma-Ray
Response
Using a Natural
Uranium Source,
Angle relative to
Window

DGA Focused Gamma Tool

The DGA Focused, or Azimuthal Gamma tool is a Tungsten collimated
version of the NGT-T tool. It is mechanically and electrically identical to
the NGT-T. A window is machined along the length of the Tungsten shield
that surrounds the detector. Only gamma rays entering from the formation,
through this window can be detected and counted. When aligned with the
tool face or other physical reference, the DGA indicates the direction from
which gamma ray intensities originate.

NGT-B Gamma Tool

for Wireline
The NGT-B Gamma Tool is a fully housed 1 11/16 OD, wireline logging tool. It is
available to operate with the CBG high speed digital telemetry or as an analog, pulse
output tool. The NGT-B incorporates the standard GO single-pin interface. Titanium
housings and subs not only provides maximum protection in sour-gas environments, but
minimizes attenuation of gamma rays due to the low density. Temperature compensated
electronics insure stable count rates over the full temperature range to 350F.

NGT-S Gamma Tool

for Wireline
The NGT-S Gamma Tool is the small diameter version of the NGT-B, with an OD of
1.375. Tool performance and stability are not sacriced for this slim hole version of
the NGT-B.

Calibration

Shock and Vibration Testing

CBG Gamma tools are calibrated in the laboratory using an AEA Technology KUTh
Field Verier, Product Code No. 188074, to determine the API calibration factor
for each tool. The nuclides described below are carefully chosen and combined to
closely approximate the proper ratios as found in the KUTh API Calibration Test
Pits located at the University of Houston, Houston Texas.

Shock and Vibration testing is routinely employed to insure that environmental

specications are being met as well as for troubleshooting some repairs. CBG
uses the Vibration Test Systems equipment, in house to perform these tests.
Tests are performed to meet tool specications of 50-300 Hz and 30G.

Nuclides
Natural Thorium (Th-232)
Natural Uranium (U-238)
Natural Potassium (K-40)

Content

Activity

90ppm
40ppm
11.7%

0.168 uCi
0.233 uCi
1.685 uCi

Temperature Stability
CBG Gamma Tools are fully rated to 350F, with a survival rating up to 400F.
Electronic circuits are temperature compensated to maintain consistent count rates.
Each tool, new and repaired, is logged in the laboratory from room temperature to
350 and back to insure a count rate stability of no less than 95%.

Service and Repair

All tools are 100% assembled and tested by CBG. Each component of the tool
can be readily repaired or replaced. CBG has developed a reputation for fast turn
around times when service is required. The proprietary detector assembly allows
access to the scintillator crystal and photomultiplier tube for troubleshooting and
replacement, without having to send the entire assembly away to a third party for
repair. Components, assembly and test procedures are continually updated by
CBG to insure the most accurate and reliable tool on the market today!

Custom Designs
CBG will work with your Engineers to develop a customized gamma tool
design for your specic application. We have developed numerous designs for
companies that require electrical and/or mechanical changes from our standard
products.

Typical Temperature Stability of Count Rate

NGT-T MWD Gamma-ray Tool

Pa g e 3 o f 4

2601 McHale Court

Suite 145
Austin, Texas 78758

Tel: 512-491-7541
Fax: 512-491-7561
www.cbgcorp.com

Gamma-Ray Tools

For Geosteering, MWD and Wireline Logging

CBG Gamma-Ray Tool Specifications

NGT-T

w/out housing assembly

Application

NGT-T

with housing assembly

NGT-CS

DGA

NGT-B

FOCUSED GAMMA

NGT-S

Geosteering/MWD

Geosteering/MWD

Geosteering/MWD

Geosteering/MWD

Wireline/Production

Wireline/Production

Diameter (OD)

1.36

1.875

1.050

1.30

1.6875

1.375

Length (make up)

13.6

34.05

18.83

13.6

22.25

25.2

Weight

1.7 lb.

15.0 lb.

1.5 lb.

3.0 lb.

6.0 lb.

4.0 lb.

Operating Temp.

-77 to +350 F.

-77 to +350 F.

-77 to +350 F

-77 to +350 F

-77 to +350 F

-77 to +350 F

End Connectors

MDM-15 Pin

200C, 10 Pin GE

MDM-15 Pin

MDM-15 Pin

GO Single Pin

GO Single Pin

Mechanical

Material

BeCu

Ti-6Al-4V

Ti-6Al-4V

Pressure

18,000 PSI

18,000 PSI

18,000 PSI

Performance
Sensitivity

2.0 Counts per API

1.7 Counts per API

1.8 Counts per API

0.6 Counts per API

1.7 Counts per API

1.5 Counts per API

Accuracy

+/- 5% to 300 F.
+/- 10% to 350 F.

+/- 5% to 300 F.
+/- 10% to 350 F.

+/- 5% to 300 F.
+/- 10% to 350 F.

+/- 5% to 300 F.
+/- 10% to 350 F.

+/- 5% to 300 F.
+/- 10% to 350 F

+/- 5% to 300 F.
+/- 10% to 350 F

Resolution
(Thin-Bed, 8 hole
diameter, 50% points)

6.8

6.8

6.8

8.8

8.8

8.8

Survival Temp.

400 F.

400 F.

400 F.

400 F.

400 F.

400 F.

Max Heat/Cool

5 F./Minute

5 F./Minute

5 F./Minute

5 F./Minute

5 F./Minute

5 F./Minute

Vibration (3 axis)
50-300 Hz
Random

30 G.
30 G.

30 G.
30 G.

30 G.
30 G.

30 G.
30 G.

30 G.
30 G.

30 G.
30 G.

Shock (Z-axis)

500 G., 0.5 mS.

500 G., 0.5 mS.

500 G., 0.5 mS.

500 G., 0.5 mS.

250 G., 0.5 mS.

250 G., 0.5 mS.

Shock (Y-axis)

1000 G., 0.5mS.

1000 G., 0.5mS.

1000 G., 0.5mS.

1000 G., 0.5mS.

500 G., 0.5mS.

500 G., 0.5mS.

Input Voltage

22-30 Volts

22-30 Volts

22-30 Volts

22-30 Volts

46-48 Volts

46-48 Volts

Input Current

18-14 mA.
(constant power)

18-14 mA.
(constant power)

18-14 mA.
(constant power)

18-14 mA.
(constant power)

20-23 mA

20-23 mA

Maximum Voltage

31.5 Volts

31.5 Volts

31.5 Volts

31.5 Volts

50 Volts

50 Volts

+5V to 0V, 2(+/-0.5)

microseconds

+5V to 0V, 2(+/-0.5)

microseconds

+5V to 0V, 2(+/-0.5)

microseconds

+5V to 0V, 2(+/-0.5)

microseconds

CBG Telemetry /Pulse

CBG Telemetry / Pulse

Environmental

Power Requirements

Output Signal
Pulse

For more information, call us today at 512-491-7541

Pa g e 4 o f 4

2601 McHale Court

Suite 145
Austin, Texas 78758

Tel: 512-491-7541
Fax: 512-491-7561
www.cbgcorp.com